Chiral-at-Metal Racemization of Tris-Chelated Octahedral Complexes: A DFT Case Study of Al(acnac)3.

Configurationally stable chiral M(α-chel)3 complexes (α-chel = asymmetric chelating ligand) play a central role in asymmetric catalysis, yet the fundamental processes governing their racemization remain poorly understood. Density functional theory calculations at the ωB97X-D/def2tzvpp and ωB97X-D/6-311++G(d,p) levels were employed to investigate the racemization and isomerization mechanisms of Al(acnac)3 as a prototypical M(α-chel)3 system. Of the mer and fac enantiomeric isomers the mer form is favored by 1.7 kcal mol-1. Their isomerization is kinetically challenging with a 26.7 kcal mol-1 high-energy d,l-cis pathway. Racemization of mer-Al(acnac)3 via either trans or d,l-trans transition structures occur with slightly lower barriers of approximately 20 kcal mol-1 for both. Methyl substitution of all N-chelate sites enhances the stabilization of the mer isomer over the fac isomer to 5.8 kcal mol-1 and inhibits access to the cis, trans, and d,l-cis transition structures due to steric repulsion. This leaves the d,l-trans pathway as the sole channel for Δ-mer ⇌ Λ-mer racemization, albeit with a very substantial barrier of 28.1 kcal mol-1. These results establish a unified mechanistic and topological framework for racemization in M(α-chel)3 complexes and provide design principles for developing robust chiral-at-metal architectures.